The present invention relates to electron guns for producing spiral electron beams, and also to gyrotron devices including such electron guns.
The gyrotron device, also called electron cyclotron maser, is a newly-developed, high-power millimeter-wave device which utilizes an electron gun producing a spiral electron beam. The millimeter-wave electro-magnetic radiations generated by this device result from the phase-bunching of the electrons which are rotated in a corkscrew motion about the lines of the axially-extending magnetic field. This device has been operated in several different configurations, including an oscillator configuration wherein the electron beam is propagated through a shaped cylindrical cavity, and a travelling-wave-amplifier configuration wherein the electron beam is propagated in a hollow pipe.
The method for producing the spiral electron beam used in the first scientific demonstration of the operation of the gyrotron is commonly called the "magnetic corkscrew" technique. This technique superimposes, on an axial magnetic field, a rotating magnetic field whose spatial rotation period is the same as the spatial rotation period of the electron beam. The latter period is determined by the rotational velocity of the electrons, which is in turn determined by the magnetic field and the drift or parallel velocity of the electrons. However, the "magnetic corckscrew" technique has not been used in the current generation of high power gyrotrons because it has critical adjustment problems and requires exact spatial resonance, a very long tube, and a specific shape of the magnetic field.
A second known method for producing the spiral beam is called the "cusp" technique. This technique generates the spiral electron beam by the injection of a linear beam off-axis into a cusp to produce a magnetic field having a radial component. However, this technique also suffers from the disadvantage of critical adjustment problems since the perpendicular component of the magnetic field cannot be adjusted independently of the axial component.
The method of producing spiral electron beams most commonly used today is called the "magnetron injection" technique. It operates on the principle of crossed electric and magnetic fields such that the beam is generated initially with a transverse component. Thus, the electric field extends between the cathode and the anode, and also between the cathode and a control electrode, so that it includes both an axial component and a radial component. The magnetic field is an axial one, so that as soon as the electrons leave the cathode, they experience a crossed electric and magnetic field producing the spiral motion of the electron beam.
While the "magnetron injection" technique is the most common method today of producing spiral electron beams for gyrotrons, this technique also requires that the values of the magnetic and electrical fields be very carefully controlled, to prevent the electron orbits from becoming unstable such that the electrons leaving the cathode may return to it. In addition, the magnetron injection gun has a relatively short useful life, for the following reasons:
Generally speaking, high power electron guns operating in the "temperature-limited condition" are usually short-lived because of the contamination of the cathode surface. For long life, the cathodes must generally operate in the "space-charge-limited condition," wherein there is a sufficiently dense cloud of electrons in front of the emitting surface that the predominant source of electrons becomes a cloud rather than the cathode surface itself. Since magnetron injection guns require critical adjustment of the electric field, and since the electric field becomes unpredictable when there is a strong space charge, these guns are generally operated in the "temperature-limited condition", and therefore their lifetime is quite short, being in the tens or at most hundreds of hours. Such guns, therefore, have a very short useful life when compared to electron guns operating in the space-charge limited condition which commonly have lifetimes in the thousands of hours.
An object of the present invention is to provide a novel electron gun for producing a spiral beam which gun has advantages over the known types described above. Another object of the invention is to provide a gyrotron including the novel electron gun.